In rural area, a standing mailbox is disposed near a road which may be at a distance from a house. The mailbox may or may not be locked. In case it is lockable, the mailbox door is provided with a slot to allow insertion of the mail. The mailbox is commonly provided with an erectable arm. When a mail is inserted into the box, the postman `flags up` the arm to indicate that there is mail in the box. When the mail is withdrawn, the arm is then `flagged down`. This mechanical arrangement may be overlooked, particularly when the postman is not so co-operative or he is rushing for time, for an example, when it is drizzling.
In urban settings like a condominium or an office block, the mailboxes belonging to the occupants are placed at a convenient location such as the basement or the ground floor. Each mailbox belonging to each housing or office unit is invariably locked, in order to discourage unauthorized access to the mail. Each mailbox is provided with a slot to allow insertion of the mail. It is also common to cover the slot with a tiltable cover plate to discourage peeping. The security arrangement, however, does not allow easy knowledge whether there is mail in the box or not.
The U.S. Pat. No. 3,909,819 discloses a mailbox alarm apparatus comprising an enclosed mailbox having an openable door; and photosensitive means for initiating activation of an alarm signal in response to light coming from outside the mailbox when the door is opened. The main disadvantage of this teaching is that the door has to be opened before the alarm is being activated. In normal situation, the mailbox is locked and a slot is provided on the door to allow mail to be inserted. The door is invariably locked, to discourage unauthorized access.
The U.S. Pat. No. 4,101,877 discloses a mail delivery alarm system for activating at least one signal device located in a house when mail is deposited in a mailbox located outside the house, and for deactivating the signal device when the mail is withdrawn from the mailbox. A switch means is provided inside the mailbox and a circuit is arranged to activate the signal devices every other time the switch means is closed. An electrically engageable lock means is located in said mailbox for keeping the mailbox door shut, said lock being engaged every other time said switch means is closed, and the switch means can be manually operated for disengaging said lock means. The main disadvantage of this teaching is that the signal device is dependent on the switch means which is largely dependent on a counter device.